The present invention relates to a dry, freely flowing powder mix comprising a phase change material.
Phase change materials may be repeatedly converted between solid and liquid phases and utilize their latent heat of fusion to absorb, store and release heat or cool during such phase conversions.
These latent heats of fusion are greater than the sensible heat capacities of the materials. For example, in phase change materials, the amount of energy absorbed upon melting or released upon freezing is much greater than the amount of energy absorbed or released upon increasing or decreasing the temperature of the material over an increment of 10.degree. C.
Upon melting and freezing, per unit weight, a phase change material (PCM) absorbs and releases substantially more energy than a sensible heat storage material that is heated or cooled over the same temperature range. In contrast to a sensible heat storage material that absorbs and releases energy essentially uniformly over a broad temperature range, a phase change material absorbs and releases a large quantity of energy in the vicinity of its melting/freezing point.
Phase change materials capable of storing and releasing thermal energy have found many applications in building structures, road base materials, beverage and food containers, medical wraps, and textile applications such as garments. One of the basic problems, however, in the use of solid-to-liquid PCM's for control of temperature, is containment. That is, for heat transfer efficiency as well as safety purposes, it is undesirable to have a thick block or agglomeration of solid phase PCM below the PCM melting point. Similarly, when above the melting point, PCM in liquid phase can be problematic. For instance, building panels containing liquid phase PCM have proven deficient. In one such PCM-containing panel, carpenters reported that a liquid phase PCM leaked out of the panel when nails were driven through it.
In those situations in which medical hot or cold packs containing PCMs are used, a solid phase agglomerate of PCM below its melting point renders the structure unwieldy and incapable of conforming about the required body part to achieve the desired heating or cooling function.
Accordingly applicant has developed a series of PCM containment systems. These are represented by U.S. Pat. Nos. 4,617,332, 4,711,813, 4,797,160, 4,908,166, and 5,053,446, all assigned to the same assignee as the present invention. However, none of the containment means disclosed in those patents involve silica. Still, the broad idea of using silica as a suspension medium for PCMs in building blocks is not new. For instance, see U.S. Pat. No. 4,259,401 (Charoudi et al) wherein this concept is disclosed at column 21, line 60 et seq. Also, Johnson et al in U.S. Pat. No. 4,237,023 discloses incorporating fumed silicon dioxide with inorganic phase-change salts which are capable of forming salt hydrates in the presence of water and Chang in U.S. Pat. No. 4,292,189 discloses a phase change energy storage system based on a combination of two inorganic salts together with nucleating and thickening agents including silicas. Finally, Allen, U.S. Pat. No. 4,008,170 describes a powdered product prepared by the vapor phase hydrolysis of a silicon compound reacted with liquid water. However, the dry water in Allen is not used as a phase change material for the storing and releasing of thermal energy.
Besides, the prior art does not suggest utilization of the combination of the preferred silica having the recited particle size and the PCM/silica weight ratios herein required in order to result in a dry, conformable, powder-like, PCM containing composition that may be useful in widespread environments.
Accordingly, it is an object of the invention to provide a conformable, powder-like PCM-matrix composite that will not liquefy upon heating of the PCM above its melting point and will not form a rigid solid at temperatures below the melting point. In other words, it is desirable to find a new method of containment for the PCM wherein, when above or below its melting point, the PCM-matrix structure will be in the form of a soft, conformable configuration like a sand pack.